Field of the Disclosure
The present invention relates to fuel cells and batteries and in particular to flow batteries for the rechargeable generation of electricity.
Description of the Related Art
Flow batteries are rechargeable fuel cells in which electrolyte containing one or more dissolved electro-active species flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in one or more storage tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known. Flow batteries can be rapidly “recharged” by replacing the electrolyte liquid; the spent material being simultaneously recovered and reenergized.
Various classes of flow batteries exist. One type of flow battery is the redox (reduction-oxidation) flow battery, which is a reversible fuel cell in which all electro-active components are dissolved in the electrolyte. The redox flow battery is a secondary fuel cell or regenerative fuel cell, meaning that it may be recharged. Essentially chemical energy is converted to electrical energy, and by passing a reverse current, the battery may be recharged.
Flow batteries where one or more electro-active components are deposited as a solid layer are known as hybrid flow batteries. Such electrochemical cells contain one battery electrode (solid phase) and one fuel cell electrode. An example of a hybrid flow battery is the zinc-bromine battery that may be used for energy storage, where the zinc anode is the solid phase electrode and dissolved bromine is the cathode component.
The main difference between the Redox type and the hybrid type of flow batteries is that as with other fuel cells, the energy of the redox flow battery is fully decoupled from the power because the energy is related to the electrolyte volume, i.e. to the tank size, whereas the power is determined by the electrode area. However, as with typical batteries, the energy of the hybrid flow battery is limited by the size of the battery electrodes.
In the simplest form a flow battery or cell comprises two solutions separated by an ion selective membrane, where each solution is in contact with a catalytic electrode and is capable of being pumped by or through that electrode (the flow) for storage in separate tanks.
Traditional hybrid flow batteries are a subclass of lower cost flow batteries in which only one redox solution is used; the other electrode comprising an active metal that is precipitated on charge from the salt solution. One example of a hybrid flow battery is iron/ferric chloride using acidic solutions in which iron is deposited at the anode current collector on charge. Other examples include zinc-bromine, zinc-cerium and lead-acid flow batteries.
U.S. Pat. No. 4,180,623 to Adams (Lockheed) describes an example of a hybrid flow battery based on zinc/sodium ferricyanide, that uses alkaline solutions. Here zinc is deposited at the anode current collector on charge. On discharge, the zinc dissolves into solution.
Hybrid flow battery systems still require two pumps and two tanks and often have limited cycle life or poor energy efficiency due to metal dendrite formation and excessive parasitic hydrogen evolution on charging.